1. Field of the Invention
This invention relates to a hydrophilic porous membrane used for, for example, a filter of a transfusion filter for medical use and a process for preparing the same.
2. Description of the Prior Art
As a material for this kind of porous membrane, a fluorine-containing resin has been known. The fluorine-containing resin is excellent in physical strength, chemical stability to acid, alkali and organic solvents, and resistance to radiation such as gamma ray. Therefore, it has been considered that a porous membrane prepared by using the above resin is optimum one for a filter for medical use since gamma ray sterilization is possible.
Since the porous membrane comprising the above fluorine-containing resin is hydrophobic, for introducing water in fine pore portions, the membrane is to be made hydrophilic. As a method for making a hydrophilic material, there have heretofore been known such methods as using a hydrophobic porous membrane as a base material, with coating a surfactant on the surface of the base material, subjecting a hydrophilic monomer to graft polymerization, and the method utilizing de-(hydrogen fluoride) reaction by strong base treatment, to introduce a polar group to effect hydrophilic treatment.
However, in the above conventional methods of making a hydrophilic material, the following problems are involved.
That is, as a method of coating a surfactant, unless a substantial amount of the surfactant amounting to about 10% by weight based on the porous membrane base material is coated, sufficient hydrophilic property could not generally be endowed, if the porous membrane which is made hydrophilic according to this method were used as a filter, it would not be practical because the surfactant has generally high water solubility, foaming and soluble material are likely to be generated. For example, when foaming is caused at inner portion of the filter, air is intertwined on the surface of the membrane and the contacted portion loses a function as a filter membrane so that effective area which can be filtered become restricted. Further, the surfactant does not have sufficient resistance to gamma ray so that sterilization of the filter should be carried out by using ethylene oxide gas. However, the method involves the problem that the gas remains after sterilization.
Next, in the method of graft polymerization, procedures of providing hydrophilic property are complicated and the hydrophilic porous membrane obtained by the method is generally likely to be brittle. Also, due to swelling of graft chain at inner portion of the porous membrane, there is a tendency of becoming small in pore size and low in the amount of permeated water. Moreover, the problem of remaining a monomer(s) is caused.
Further, in the method of utilizing the reaction of removing hydrogen fluoride using strong base treatment, there are problems that processing of waste liquor is difficult, the hydrophilic porous membrane becomes brittle and coloring is caused due to formation of conjugated double bond.